Plasma treatment is commonly applied for modifying the surface properties of workpieces used in applications relating to integrated circuits, electronic packages, and printed circuit boards. Plasma treatment systems are configured to produce a direct plasma from a process gas and expose a surface of a substrate or workpiece with generated active species from the direct plasma to remove surface atoms by physical sputtering, chemically-assisted sputtering, or chemical reactions. The physical or chemical action may be used to condition the surface to improve properties such as adhesion, to selectively remove an extraneous surface layer of a process material, or to clean undesired contaminants from the surface. Plasma treatment is used in electronics packaging, for example, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, improving wire bond strength, ensuring void free underfill, removing oxides, enhancing die attach, and improving adhesion for encapsulation.
Plasma treatment systems may be integrated into in-line and cluster systems or batch processes in which groups of workpieces are processed by successive plasma exposures or processing cycles. Workpieces may be supplied by various methods, including delivery in a magazine, individual delivery by a conveyer transport system, or manual insertion into the process chamber. Plasma treatment systems may also be provided with automated robotic manipulators that coordinate workpiece exchange into and out of the process chamber for plasma processing operations.
Conventional plasma treatment systems have failed to provide adequate process uniformity across the surface of individual workpieces. The plasma density must be precisely and accurately controlled at all positions on the surface of the workpiece so that it is substantially uniform across the surface. Critical parameters for controlling the uniformity of the plasma include the spatial uniformity of the excitation power and the dispersion of the process gas. A non-uniform plasma density at the surface of the workpiece degrades process reliability and reduces the process yield. To achieve workpiece-to-workpiece uniformity, the process gas must be evenly dispersed and uniformly ionized by the excitation power so that the flux of active species is spatially uniform across the surface of the workpiece.
Conventional plasma treatment systems have likewise failed to achieve adequate reproducibility of the plasma treatment between successive batches of workpieces. Batch-to-batch reproducibility depends on the precise control of process variables and parameters so that successive workpieces are exposed to substantially identical plasma conditions. Moreover, conventional plasma treatment systems are incapable of rapidly processing workpieces with a throughput amenable to automated process lines or fabrication requirements. System throughput and uniformity of the plasma treatment must be maximized for reducing production costs.
Conventional in-line plasma treatment systems also lack the ability to generate a downstream-type plasma that is substantially free of ions, electrons and light in at least the visible region of the electromagnetic spectrum. As is well-known, a direct plasma is a combination of multiple different species including ions and electrons that have a net charge and source gas molecules and free radicals that are neutral. Free radicals are gas molecules that are nearly ionized yet retain their full complement of electrons so they are neither positively nor negatively charged. Workpieces may be processed with a direct plasma containing all plasma species or with a downstream-type plasma including primarily free radicals. Processing workpieces with a direct plasma promotes treatment with both physical action due to ion and electron bombardment and chemical action arising from surface interaction of the free radicals. On the other hand, processing with the downstream-type plasma involves primarily chemical action.
Conventional plasma treatment systems generally include a fixed dimension plasma chamber and a substrate support in the plasma chamber that holds the workpiece at a fixed position between opposed treatment electrodes. Because the workpiece is located at a fixed position, the surface to be plasma treated is likewise spaced relative to the treatment electrodes. The fixed position is chosen to provide a spacing effective to provide an effective plasma treatment for workpieces of a given thickness. It follows that, as the thickness of the workpiece being treated in the system changes, the location of the surface is no longer at the desired fixed position so that the efficiency of the plasma treatment may be reduced. Therefore, conventional plasma treatment systems are ill equipped to accommodate changes in workpiece thickness.
There is thus a need for an in-line plasma treatment system that can provide a downstream-type plasma for treating workpieces in the plasma chamber and that can accommodate workpieces of differing thickness while maintaining an effective treatment efficiency.